On Thursday, voters in Britain head to the polls for a historic referendum (not to mention the first local elections test for the coalition), the first national referendum since 1975, when two-thirds of the British public endorsed staying in the European Economic Community. Perhaps nearly as radical, voters get to choose whether to scrap their first-past-the-post voting system in favor of what’s called alternative vote, in which, as I explain in my article for Britannica, voters can rank candidates in order of preference, ensuring that a candidate who wins receives majority support. (According to recent polls, voters are set to reject the change.)

Why is this referendum on offer? The roots of the vote date to last year’s indecisive election, when the Conservative Party won the most seats in the House of Commons but fell shy of a majority. To entice the Liberal Democrats into a coalition government, Conservative leader (and now prime minister) David Cameron offered Liberal Democrats chief Nick Clegg (now deputy prime minister) a referendum on alternative vote, which fell short of the Lib Dems’ goal of full proportional representation. Indeed, in April 2010 Clegg had dismissed alternative vote as a “miserable little compromise,” instead favoring “alternative vote plus,” which was proposed in 1998 by a commission on the voting system heading by Lord Jenkins and which would have brought more proportionality by electing some percentage of MPs via regional party lists.

The campaign has tested the coalition between Cameron and Clegg, and Clegg has even said that the “No” campaign, in which Cameron resides, is guilty of “misinformation.” Still, the pair, who are still said to get along very well, said that the coalition would continue even after the campaign was over, no matter which side wins. The Economist, normally a supporter of constitutional reform, has weighed in, lending its weight, perhaps surprisingly, to the “No” campaign, though in a backhanded fashion. An editorial in the influential paper says that “British democracy is in need of repair work” and “[f]laws in the current system of first-past-the-post (FPTP) have contributed to a dangerous sense that British voters have too little sway over those who govern them,” but that the reform on offer is not proportional enough and that the “referendum is a disappointment.”

The campaign turned “nasty” months ago, devolving into charges and counter-charges of lying—with Clegg even accusing the prime minister himself of lies. While the Lib Dems and Conservatives have been split, with most Lib Dems in favor of AV and most Conservatives against, Labour has been able to sit back, its division masked somewhat by the coalition’s infighting. Labour leader Ed Miliband has endorsed alternative vote and called on Labour voters to back the reform (he has recently charged Cameron with relying on “smears and fears” to win the vote), but many Labour MPs and rank-and-file voters have opposed the change. Indeed, John Healey, the shadow secretary of state for health, one of three shadow ministers opposed to AV, penned a piece in the Independent in March in which he laid out the main arguments of the “No” campaign: that AV would benefit extremist supporters (a charge that supporters dismiss), saying that Britain might “witness the unedifying prospect of the major parties chasing transfers from racist, bigoted, eccentric and single-issue candidates,” and that it would produce hung Parliaments “more often than not,” taking out of the hands of voters the ability to “thrown one government out and put another in.” (The BBC has a good guide to where the parties stand.)

Before the charges and counter-charges were flying, however, we at Britannica Blog got the reasoned scoop from leaders of the Yes and No campaigns on why voters should support or not the radical change on offer on Thursday. (See full interviews with Matthew Elliot of NO2AV and with Peter Facey of Unlock Democracy and a supporter of AV.)

Why not AV? Among what Elliott told us about the negative effects of AV:

Ignoring the most immediate problem of having to explain a complicated new voting system to the country; it would mean that in constituencies where the minor/extreme parties are strong, their voters would effectively decide the result through their second preferences. As a result we could see some candidates associating themselves with parties like the BNP. Similarly, in the ‘big three’ party marginal seats, candidates may be tempted to adopt a more centrist tone, removing the debate and colour from party politics. Britain would also be signing up for permanent coalition governments and the uncertainty that brings. Although our present Government has handled this situation well, they have been held together through a dire need to tackle the financial crisis. Despite this, there are many Conservative and Liberal Democrat voters who are angry that either side has partnered with the other. This would be a permanent feature of the British political scene if AV was passed. The decision of who runs the country would be made in Westminster backrooms by the political elite and not by the voters. Minority parties – or even 2 or 3 MPs as is happening now in Australia – would command undue influence in policy-making as successive Governments appealed to fringe MPs to try and gain extra votes in the House of Commons. Probably worst of all, the introduction of AV could signal the end of the political reforms that are really needed – recall of MPs, open primaries and a tougher Freedom of Information Act, all of which would really shine a light on our elected representatives.

Facey countered with the primary benefits of AV:

By forcing MPs to get the support of more than half of their constituents, AV forces politicians to work harder to earn and keep their support. They can no longer take local support for granted. By allowing voters the option of expressing a preference of candidates, AV gives people more of a say and lets them vote for what they really want. There is no need to vote tactically and vote against a candidate rather than for the candidate who would be your first choice.